Re: Setting Terminal Line Charcateristics

From: John Santos (john_at_egh.com)
Date: 11/21/05

  • Next message: Chris Scheers: "Re: No more SETI on VMS ?" 
    Date: Mon, 21 Nov 2005 03:19:27 GMT

    Bob Hassinger wrote:
    > John.Martin_At_Home@BTInternet.com wrote:
    >
    >> I wish to connect a terminal line from my MicroVax3100 (VMS 7.2) to my
    >> PDP8i computer its terminal port runs at 100 baud
    >
    >
    > The standard PDP-8/I current loop interface ran/runs at 110 baud. If it
    > is the standard hardware, and if it is operating with a standard ASR33
    > then it is highly likely you are looking at the 110 baud.
    >
    > with 2 stop bits. To
    >
    >> change the PDP8i to 1 stop bit requires hardware changes which I do not
    >> want to do as I also use it with a ASR33 Teletype which requires the 2
    >> stop bits. I have built an interface that converts the PDP8i 20mA
    >> current loop to/from RS232.
    >
    >
    > I built one too - in 1968.
    >
    > I know my interface is good as it all works
    >
    >> connected to a PC where it is a simple matter to specify 2 stop bits.
    >>
    >> The Set Terminal command will let me set the baud speed but how do I
    >> set the 2 Stop Bits. I have successfully connected my PDP8e to the Vax
    >> but this expects 1 stop bit.
    >>
    >> Any help would be appreciated.
    >
    >
    > Are you sure you need to worry about the two stop bits? The VAX is not
    > going to care that you are sending two stop bits toward it (there is
    > just a bit time of no data between each character.
    >
    > Have you determined that is does not work if you only send one stop bit
    > towards the 8/I? Its been more than 35 years since I looked at the
    > schematic for that interface, but there is a good it may be satisfied
    > with one stop bit too.

    It might be possible to play games with the character size. IIRC, the
    line idles a 1, and the start and stop bits are 0's. (Could easily have
    that backwards.) To get 2 stop bits, it may be possible to either set
    the character size 1 bit larger (8 bits, no parity) and always send a 0
    as the eighth bit, or set it 7 bits, space parity. If the stop bit has
    to be a 1, then 7 bits, mark parity, or 8 bits and filter the output
    to force the 8th bit to a 1. C-Kermit can do this (according to its
    help), though I don't know if it works on VMS. See SET PARITY and SET
    SERIAL in C-Kermit.

    IIRC, the standard teletype setting was 7 bits, no parity, 2 stop
    bits. The low bit is sent first. So the PDP-8 should see the 8th
    data bit (or the parity bit if parity is enabled, when it is expecting
    the 1st stop bit, so all you need to do to fake it out is send the
    right bit (0 or 1, depending on what stop bits are supposed to be)
    as the parity bit (i.e. Space parity) or as the eighth data bit.

    It is also possible you don't need to do anything at all.... I've
    seen many terminals and terminal interfaces that automatically set 2
    stop bits if the baud rate is 110 or below (and 1.5 stop bits for 150?)
    Maybe VMS does this and it has been lost in the mists of time.

    > Bob Hassinger
    >
    >>
    >> John
    >> 

    -- 
John Santos
Evans Griffiths & Hart, Inc.
781-861-0670 ext 539
  • Next message: Chris Scheers: "Re: No more SETI on VMS ?"

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